Recovery of products from electrochemical fluorination

ABSTRACT

FLUORINATED PRODUCTS ARE RECOVERED FROM AN EFFLUENT STREAM FROM AN ELECTROLYTIC CELL IN AN ELECTROCHEMICAL FLUORINATION PROCESS BY EMPLOYING A COMBINATION OF STEPS COMPRISING COOLING SAID EFFLUENT STREAM TO A TEMPERATURE SUFFICIENT TO CONDENSE HYDROGEN FLUORIDE AND HIGHER BOILING CELL PRODUCTS CONTAINED IN SAID EFFLUENT STREAM, CONTACTING THE NONCONDENSED PORTION OF SAID COOLED EFFUENT STREAM WITH AN ABSORBENT SELECTED FROM THE GROUP CONSISTING OF (A) THE FLUORINATABLE FEEDSTOCK TO THE ELECTROLYTIC CELL, (B) A HIGH BOILING BY-PRODUCT FRACTION PRODUCED AS A BYPRODUCT IN THE FLUORINATION REACTION, AND (C) MIXTURES OF SAID (A) AND SAID (B), AND RECOVERING FLUORINATED PRODUCTS FROM THE RESULTING RICH ABSORBENT AND THE CONDENSED PORTION OF SAID COOLED EFFLUENT STREAM.

6 91 F'. N. RUEHLEN 3 50 7 mcovm 0F PRODUCTS FROM ELECTROCHEMICAL FLUORINATION March 21, 1972 Fiied Aug. 25, 1969 k l M mi m m m S m E m u w n w H w M l O Y m O N N /v N FE rw@ fom o mw Y N@ Nm\ v\ om( mj 1 i G G w w w qm l l S wml, I V w k W x w f m .mm om; N w E Q m m Hom .wv N N FQ @mi @mi m @mL EL @mL NL m Ov\ A L L o .\.`a` v@ o@ msm mooz oP\ mm @v msm muovi/ OPS JVA WAIVA ABSTRACT F THE DIscLosURE Fluorinate'd products are recovered from an ellluent stream froman electrolytic cell in an electrochemical iluorinatior process byemploying a combination of steps comprising cooling said effluent stream to a temperature sulllcientto condense hydrogen-fluoride and higher boiling cell 'products'contained inlsaid eflluent stream, contacting thenoncondens'ed portion of said`cooled ellluent stream with an absorbent lselectedrfrom the group consisting of (a) the iluorinatable feedstock to the electrolytic cell, (b) a high boiling by-product fraction produced as a byproduct in the iluorination reaction, and (c) mixtures of said (a) and s aid (b),-and recovering fluorinated products fromthe resulting rich absorbent and the condensed portion of said cooled effluent stream.

This invention relates to electrochemical fluorination. In one aspect the invention relates tothe recovery of fluorinated products inan electrochemical iluorination process.

Electrochemical iluorination processes for preparing or converting a wide variety of feedstocks into desirable lluorinated products are well known in the art. Generally speaking, these processes usually involve immersing an electrode element in an electrolyte and passing an electric current through said electrolyte between said electrode and an oppositely charged element, eg., either another electrodelimmersed in said electrolyteor the cell body which can serve as said other element or electrode. In recent year-s electrochemical tluorination processes have been developed wherein porous anode elements are employed. In one process employing a porous anode element a feedstock to be converted is passed through the porous anode into'rthe main body of the electrolyte.

'Recently it has been discovered that the reaction in an electrochemical lluorination process can be carried out withinthe contines, e.g., within the pores, of the porous anode itself. Ihis type of operation is of particular utility withmany feedstocks because it provides or makes pos! sible` av simple one-step route,` at relatively high conversions, 'to produce partially fluorinated productswhich had previously `been difficult to obtain. This process also allows operation at` high conversions without substantial formation of cleavage productswhich are generally produced by they older` methods when operating at high conversions. The'jfeedstock to be fluorinated can be introduced into the pores of theporous anode at a point near its bottom' and-.the fluorinated mixture removed from said pores at the top of the anode, generally above the electrolyte level.

Passage of the feedstock into the bulk of the electrolyte isV thus avoided. i y' One'problemk which is common to all of the abovedescribed electrochemical lluorination processes is the recovery of the fluorinated products from the cell etlluent stream. 1n many instances when electrolyte comprising a current-conducting hydrogen fluoride is employed, said cell etlluentvwill usually contain hydrogen which is pro-` duced as a cathode product, some hydrogen fluoride which vaporizes from the electrolyte, various fluorinated products produced from the feedstock, and unreacted feedstock.

Generally Vspeaking,...'inrrnost instances said ,fluorinated nited 4States, Patent Ollice 3,650,9l7 Patented Mar. 2l, 1972 products will include light-ends comprising fluorinated products which are lower boiling than the feedstock, the desired fluorinated product or products, and a small amount of a iluorinated high boiling by-product. Separation of the hydrogen and hydrogen fluoride from the cell eilluent without loss of the light-ends fluorinated products is particularly troublesome.

The present invention provides a solution for the abovedescribed problems. It has now been discovered that hydrogen fluoride and hydrogen can be efllciently separated from the light-ends fluorinated products and other fluorinated products by a combination of steps which, broadly speaking, comprises cooling the cell effluent stream to a temperature which is sufficient to condense hydrogen fluoride and higher boiling materials contained in said eflluent stream, contacting the noncondensed portion of said cell eflluent stream in an absorption zone with an absorbent selected from the group consisting of (a) the lluorinatable feedstock to the cell, (b) a fluorinated high boiling byproduct fraction produced in the fluorination process, and (c) mixtures of said (a) and said (b), and then recovering fluorinated products from the resulting rich absorbent and the condensed portion of said cooled effluent stream.

An object of this invention is to provide an improved electrochemical iluorination process. Another object of this invention is to provide a method for recovering fluorinated products in an electrochemical lluorination process. Another object of the invention is to provide a method for separating hydrogen, hydrogen fluoride, and fluorinated products from a cell ellluent stream in an electrochemical luorination process with minimum loss of valuable fluorinated products. Other aspects, objects, and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided in a process for the iluorination of a lluorinatable organic compound feedstock wherein, an electric current is passed through a current-conducting essentially anhydrous liquid hydrogen fluoride electrolyte contained in an electrolysis cell provided with a cathode and an anode, a feedstock is passed into said cell and into contact with said anode and at least partially tluorinated, and fluorinated product is recovered from an effluent stream withdrawn from said cell, the improvement comprising: passing said effluent stream to a cooling zone and therein cooling said eilluent stream to a temperature sufllcient to condense hydrogen fluoride and higher boiling materials contained in said etlluent stream; passing said cooled etlluent stream to a first separation zone and therein effecting a separation between condensed hydrogen fluoride and other condensed etlluent components including fluorinated products and any unreacted feedstock; passing the noncondensed portion of said cooled effluent stream to an absorption zone and therein contacting same with an absorbent selected from the group consisting of (a) said feedstock, (b) a high boiling byproduct fraction produced in said fluorination step and (c) mixtures of said (a) and (b); and recovering fluorinated products from the resulting rich absorbent and said other condensed effluent components.

A number of advantages are'realized or obtained in the practice of the invention. One important advantage is that no stripping column is required to separate from the absorbent the absorbed fluorinated and/or untluorinated products which it is desirable to recycle to the cell, because the absorbent employed is a normal component ofthe cell feed stream and/or the cell effluent stream. vAnother advantage is that said absorbent, being a normal component of the cellfeed stream and/or the cell effluent stream,.

introduces no contaminants into said cell eilluent stream.

Still another advantage is that by properk control ofthe operating conditions in the absorption step of the invention, losses of fluorinated products can be reduced to the bare minimum, e.g., unavoidable mechanical losses. Thus, the invention provides a truly efficient method for the recovery of fluorinated products from a cell eflluent stream in electrochemical fluorination processes.

The invention is applicable to any electrochemical fluorination process employing an electrolyte comprising essentially anhydrous hydrogen fluoride. The invention is particularly applicable to electrochemical fluorination processes in which porous anodes are employed. In one presently preferred process, a current-conducting, essentially anhydrous, liquid hydrogen fluoride is electrolyzed in an electrolysis cell provided with a cathode and a porous anode (preferably porous carbon), a fluorinatable feedstock is introduced into the pores of said anode and at least a portion of said feedstock is at least partially fluorinated within the pores of said anode, and fluorinated products are recovered from a cell eflluent stream.

Very few organic compounds are resistant to fluorination. Consequently, a wide variety of feed materials, both normally liquid and normally gaseous compounds, can be used as feedstocks in said process. Organic cornpounds which are normally gaseous or which can be introduced in gaseous state into the pores of a porous anode under the conditions employed in the electrolysis cell, and which are capable of reacting to form fluorinated products, are presently preferred as starting materials. Generally speaking, desirable organic starting materials which can be used are those containing from 2 to 8, preferably 2 to 6, carbon atoms per molecule. However, reactants which contain less than 2 or more than 6 or 8 carbon atoms can also be used. Some general types of organic starting materials which can be used include. among others, the following: alkanes, alkenes, alkynes, amines, ethers, esters, mercaptans, nitriles, alcohols, aromatic compounds, and partially halogenated compounds of both the aliphatic and aromatic series. It will be understood that the above-named types of compounds can be either straight chain, branched chain, or cyclic compounds.

One group of presently preferred starting materials are the normally gaseous organic compounds, and particularly the saturated and unsaturated hydrocarbons, containing from 2 to 4 carbon atoms per molecule. Normally liquid feedstocks which can be vaporized under cell operating conditions are also preferred starting materials. Some examples of these are methane, ethane, propane, butane, isobutane, ethylene, propylene, butene-2, acetylene, propyne, butyne-l, butadiene, and the like, and mixtures thereof.

One presently more preferred class of starting materials for use in the practice of the invention includes the fluorinatable, partially halogenated compounds having a boiling point higher than at least the major portion of the fluorinated products obtained therefrom. In said halogen-containing feedstocks, the halogen can be any of the halogens, chlorine, bromine, iodine, or fluorine. Preferably, the halogen is one other than fluorine. Partially chlorinated hydrocarbons have been found particularly useful. Examples of said compounds include, among others, the following: mono, di, tri, and tetrachloroethanes; monofluoro, mono, di, tri, and tetrachloroethanes; difluoro, mono, di, and trichloroethanes; trifluoro, mono, and dichloroethanes; mono, di, tri, and tetrabromoethanes; mono, di, tri, and tetradiodoethanes; etc. Thus, applicable compounds include: methyl chloride; methyl fluoride; chloroform; methylene diiodide; bromoform; chlorofluoromethane; bromochloromethane; 1,2-dichloroethane; 1,1-diiodoethane; 1-bromo-2-fluoroethane; 1,1,2- trichloroethane; 1,l-dichloro-2,2difluoroethane; 1,2-dichloropropane; 1 bromo 3-iodopropane; 1chloro3 fluoropropene; 1,l-dichloro-2,3-difluoropropane; 1,1,1,2 tetrafluoropropane; 1,1 dichlorobutane; 2,3 dibromobutane, 1,1,l-trichloro-S-iodobutane; 1,4-difluorobutene- 4 2; 1,2,3-trichlorobutane; and the like, and mixtures thereof.

The hydrogen fluoride electrolyte can contain small amounts of water, such as up to about 5 weight percent. However, it is preferred that said electrolyte be essentially anhydrous, e.g., contain not more than about 0.1 weight percent water. Commercial anhydrous liquid hydrogen fluoride containing up to about 1 percent by weight of water can be used. Thus, as used herein and in the claims, unless otherwise specified, the term essentially anhydrous liquid hydrogen fluoride includes liquid hydrogen fluoride which can contain water not exceeding up to about 1 percent by weight. As the electrolysis reaction proceeds, any water contained in the hydrogen fluoride electrolyte is slowly decomposed and said electrolyte concomitantly approaches the anhydrous state. Pure anhydrous liquid hydrogen fluoride is nonconductive. To provide adequate conductivity in the electrolyte, and to reduce the hydrogen fluoride vapor pressure at cell operating conditions, an inorganic additive can be incorporated in the electrolyte. Presently preferred additives for this purpose are the alkali metal fluorides and ammonium fluoride. Said additives can be utilized in any suitable molar ratio of additive to hydrogen fluoride within the range of from 1:4.5 to 1:1, preferably 1:4 to 1:2.

Generally speaking, the fluorination process can be carried out at temperatures within the range of from 80 to 500 C. at which the vapor pressure of the electrolyte is not excessive, e.g., less than 250 mm. Hg. It is preferred to operate at temperatures such that the vapor pressure of the electrolyte is less than about 50 mm. Hg. A presently preferred range of temperature is from about 60 to about C.

Pressure substantially above or below atmospheric can be employed if desired, depending upon the vapor pressure of the electrolyte as discussed above. Generally speaking, the process is conveniently carried out at substantially atmospheric pressure.

Current densities within the range of 30 to 1000, or more, preferably 50 to 500, mililamps per square centimeter of anode geometric surface can be used. The voltage which is normally employed will vary depending upon the particular cell configuration employed and the current density desired. Under normal operating conditions, however, the cell voltage or potential will be less than that required to evolve or generate free or elemental fluorine. Voltages in the range of 4 to 12 volts are typical. Generally speaking, the maximum normal voltage will not exceed 20 volts per unit cell. The term anode geometric surface refers to the outer geometric surface area of the porous element of the anode which is exposed to the electrolyte and does not include the pore surfaces of said porous element.

Feed rates which can be employed will preferably be within the range of from 0.5 to 10 ml. per minute per square centimeter of anode geometric surface area. Since the anode can have a wide variety of geometrical shapes, which will affect the geometrical surface area, a sometimes more useful Way of expressing the feed rate is in terms of anode cross-sectional area (taken perpendicular` to the direction of flow). More preferably, the feed rate will be such that the feedstock is passed into the pores of the anode, and into contact with the fluorinating species therein, at a flow rate such that the inlet pressure of said feedstock into said pores is essentially less than the sum of (a) the pressure of the electrolyte at the level of entry of the feedstock into said pores and (b) the exit pressure of any unreacted feedstock and fluorinated products from said pores into the electrolyte. Said exit pressure is defined as the pressure required to form a bubble on the outer surface of the anode and break said bubble away from said surface. rSaid exit pressure is independent of electrolyte pressure. Under these preferred flow rate conditions, there is established a pressure balance between the feedstock entering the pores of the anode from one direction and electrolyte attempting to enter the pores from another and opposing direction. Essentially all of the feedstock travels' within the porousanode via the pores therein until it exits from the anode at a point above the surface of the electrolyte. Broadly speaking, the upper limit on the flow rate will be that at which Vbreakout of feedstock and/or fluorinated product begins along the immersed portion of the anode. Breakout is defined as the formation of bubbles of feedstock and/or fluorinated product on the outer' immersed surface of the anode with subsequent detachment of said bubbles wherein they pass into the mainl body of the electrolyte. Broadly speaking, the lower limit ofthe feed' rate will be determined by the requirement to supply the minimum amount of feedstock sufllcient to prevent evolution of free fluorine. As a practica'l guide to those skilled in the art, the feed rates can be within the range 'of from 3 to 600, preferably 12 to 240 cc. per minute per square centimeter of cross-sectional area' (taken perpendicular to the direction of flow). Herein and in the claims, unless otherwise specified, for convenience the volumetric feed ratesl havebeen expressed in terms of gaseous volume calculated at standard conditions,'even though the feedstock may be introduced into the porous anode in liquid state.

Referring now to the` drawing, the invention will be morefully explained. In the drawing there is illustrated an electrolytic cell, denoted generally by the reference numeral 10,v comprising a cell body 13 having an anode 12 disposed'the'rein.` As here illustrated, said anode comprises-a cylinder-of porous carbon having a cavity 14 formed in the bottom thereof.`A current collector 16 is provided in intimate contact with the upper portion of said anode 12 and is' connected to the anode bus of the current'supply.v It will be noted that the upper end of anode12exten'ds above the electrolyte level 18. A circular cathode 20whichv can be a screen formed of a suitable metalu such as'a stainless's'teel, surrounds said anode 12 and is connected to the cathode bus of the current supply by a suitable lead .wire 22. Any suitable source of current and connections thereto can be employed in the practice of the invention. In Athe operation of the cell arrangement, a feedstock is introduced into the cavity portion 14 of said anode via conduit 15, travels upward through the pores of said anode, and exitsfrom the upper end of the anode above electrolyte level 18. During passage through said anode, at least a portion of the feedstock is electrochemically fluorinated. Fluorinated products together with any remaining unconverted feedstock, and possibly some electrolyte vapors, are withdrawn from the space above the electrolyte within cell 10 via conduit 24. During the introduction of said feedstock an electric current in an amount sufficient to supply the desired operating current density at the anode is passed between the anode and the cathode.

The cell eflluent stream in conduit 24 is passed'into cooler or condenser 26 wherein it is cooled to a temperature which is at least sufficient to condense the hydrogen fluoride and higher boiling cell products contained therein. Generally speaking, in most instances, said condenser preferably will be operated at a temperature within the range of from -100 to 50 C., more preferably 75 to 0 C. The pressure in condenser 26 will generally be within the range of to 200 p.s.i.g., preferably 0 to 50 p.s.i.g. However, it is within the scope of the invention Ato operate said condenser at temperatures and pressures outside said ranges so long as said temperature is sufficient to condense the hydrogen fluoride and higher boiling material contained in the cell eflluent stream. Condensate and noncondensed gases from said condenser are passed via conduit 28 into a first separation zone 30 wherein phase separations are effected between condensed hydrogen fluoride and other condensed effluent compounds including fluorinated products, unreacted feedstock and noncondensed gases. The hydrogen fluoride phase is returned to cell via conduit 31 and conduit 32. Make-up hydrogen fluoride can be supplied to the system via conduit 34. As here 6. illustrated, said first separation zone comprises a vessel wherein separation -between the two liquid phases and a gaseous phase is effected by gravity settling. However, it is within the scope of the invention to employ other phase separation means or methods, e.g., centrifuging.

The noncondensed portion of the cell effluent stream is passed from separation zone 30 via conduit 36 into absorption zone 38 wherein it is contacted countercurrently with an absorbent introduced via conduit 40. As described above, said absorbent can be at least a portion of the fresh fluorinatable feedstock from conduits 11 and 42, a fluorinated high boiling by-product fraction from conduit 44 as described further hereinafter, or a mixture of said two absorbents. Unabsorbed gases, principally hydrogen, are released from the absorber via conduit 46. Said absorber 38 in most instances will be operated at a temperature within the range of from to 100 C., preferably within the range of from -35 to 50 C., and a pressure within the range of O to 200 p.s.i.g., preferably 0 to 50 p.s.i.g. Generally speaking, in most instances, the ratio of absorbent to gas employed in absorber 38 will be Within the range of 10:1 to 1:1, preferably 6:1 to 3:1, mols of absorbent feed per mol of gas feed to absorber 38. However, it is within the scope of the invention to employ ratios of absorbent to gas outside of said ranges. As will be understood by those skilled in the art in view of this disclosure, the operating conditions in said absorber 38 will depend upon the composition of the stream 36 being treated, the operating conditions in cell 10, separator 30 and distillation column S4, etc. Thus, the above-described conditions are given by way of example only and are not to be taken as limiting on the invention. Said absorber 38 can comprise any suitable means for contacting the stream in conduit 36 with the absorbent from conduit 40, eg., a packed column, a bubble tray column, etc.

The rich absorbent is withdrawn from absorber 38 via conduit 48, combined with the condensed organic effluent withdrawn from phase separator 30 via conduit 50, and the combined stream is then passed via conduit 52 into a second separation zone. As here illustrated, said second separation zone comprises distillation or fractionation columns 54 and 56. However, it is within the scope of the invention to employ less than or more than two distillation columns in said second separation zone, depending upon the separations it is desired to effect. -It is also within the scope of the invention to employ separation methods other than fractional distillation in said second separation' zone, eig., extraction, crystallization, etc. However, fractional distillation is usually preferred.

In distillation column 54 a separation is effected between a first stream comprising unreacted feedstock and absorbent lwhich is withdrawn via conduit 58, and a second stream comprising desired fluorinated products which is withdrawn via conduit 60. In those instances where at least a portion of the fresh feedstock from conduits 11, 42 and 40 is used as absorbent in absorber 38, the stream in conduit 58 will comprise primarily unreacted feedstock and a small amount of higher boiling byproduct fraction produced in the fluorination step. Fluorinated products in said second stream in conduit 60 are passed into distillation column 56 wherein a separation into desired products is made, with said products being withdrawn via conduits -62 and 64. If desired, depending upon the product desired and the composition of the streams in said conduits 62 and 64, either of said streams can be recycled to the fluorination cell.

Said stream in conduit 518 comprising unreacted feedstock is recycled via conduit 59 to conduit 15 for introduction into cell 10 as described above. Preferably, a portion of said stream in conduit 58 is passed via conduit 66 into a third separation Zone, here illustrated as comprising three distillation or fractionation columns. However, as will be understood by those skilled in the a'rt, said third separation zone ican comprise less than or more than three distillation columns, depending upon the separations it is desired to effect. It is also within the scope of the co .com

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10. A process according to claim 1 wherein: said feedstock is ethylene dichloride; at least the major portion of said feedstock is introduced into said absorption zone as said absorbent; rich absorbent from said absorption zone is combined with said other condensed eluent components from said iirst separation zone; the resulting combined stream is passed to a second separation zone and therein separated into a iirst stream comprising ethylene dichloride and partially fluorinated products, another stream consisting essentially of 1,2-dichlorotetrauorethane as one product of the process, and another stream consisting essentially of chloropentauoroethane as another product of the process; and at least a portion of said tirst stream is recycled to said electrolysis cell as at least a portion of said feedstock thereto.

11. A process according to claim 10 wherein: another portion of said rst stream is passed to a third separation zone and therein separated into a stream comprising 1,2-

dichloro-1,2,2-triuoroethane which is recycled to said electrolysis cell, a stream comprising ethylene dichloride which is recycled to said electrolysis cell, a stream comprising 1,1,2-trichloro-1,2,2-trifluoroethane which is recovered as another product of the process, and a stream comprising a uorinated high boiling by-product fraction.

12. A process according to claim 1 wherein said cooling zone is operated at a temperature within the range of from -100 to 50 C. and a pressure within the range of from 0 to 20() p.s.i.g.

13. A process according to claim 6 wherein said cooling zone is operated at a temperature within the range of from -100 to 50 C. and a pressure within the range of from 0 to 200 p.s.i.g.

References Cited UNITED STATES PATENTS 2,519,983 8/ 1950 Simons 204-59 3,511,761 5/1970 Childs et al.

JOHN H. MACK, Primary Examiner N. A. KAPLAN, Assistant Examinerv 

